Events During the Week of
April 13th through April 20th, 2014

Abstract: Please visit the following link for more details:http://cmb.physics.wisc.edu/journal/index.html
Please feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Le Zhang (lzhang263@wisc.edu)

Abstract: We studied the electronic and magnetic properties of iron-chalcogenide Fe$_{1+y}$Te compounds based on the multiband model, in which localized spins and itinerant electrons coexist and are coupled by Hund's rule coupling. Integrating out the conduction electrons, we computed additional couplings between localized spins similarly to the conventional Ruderman-Kittel-Kasuya-Yosida (RKKY)theory. We found that resulting RKKY-like interactions are substantial up to the third neighbors, but are negligible beyond this. We computed the magnetic phase diagram of the modified $J_1-J_2-J_3-K$ model and showed it captures the evolution of the magnetic order in Fe$_{1+y}$Te as a function of Fe excess $y$.

Abstract: In physical systems, boundaries in parameter space that separate different large-scale behavior correspond to phase transitions, where small changes in microscopic parameters lead to drastic changes in macroscopic observables. We use fine-grained data about conflict in a macaque society to ask whether this social system is located near a phase transition. We find using two models (an equilibrium Ising model and a dynamic branching process model) that the system is near but below a transition, indicating that aggression dissipates quickly enough to avoid becoming typically widespread, but not so quickly that large fights are impossible. A relation between thermodynamics and information theory shows that being near the transition implies that it is easier for an observer of fight sizes to infer changes in individual proclivities to fight. More generally, this points to the possibility of quantifying a system's collective behavior by measuring the degree to which information can percolate among different spatial scales.<br>

Abstract: The BICEP2 experiment is a cosmic microwave background (CMB) polarimeter specifically designed to search for the signal of inflationary gravitational waves in the B-mode power spectrum around l=80. BICEP2 observed from the South Pole for three seasons from 2010 to 2012. Using its full 3-year CMB data set, we report an excess of B-mode power at the angular scales of interest, inconsistent with the null hypothesis at a significance of >5σ. The observed B-mode power spectrum is well-fit by a lensed-LCDM + tensor theoretical model with tensor/scalar ratio r=0.20+0.07−0.05. We will describe the BICEP2 instrument, observing strategy, data analysis, and results. We will also describe Keck Array, a successor to BICEP2, which is actively collecting sky data at both 100 and 150 GHz at high sensitivity for improved constraints on galactic foregrounds.

Abstract: One of the most important discoveries of the Fermi Gamma-ray Space Telescope is the detection of two giant bubbles extending 50 degrees above and below the Galactic center (GC). The symmetry about the GC of the Fermi bubbles suggests some episode of energy injection from the GC, possibly related to past jet activity of the central active galactic nuclei (AGN). Thanks to the proximity to the GC, the Fermi Bubbles are excellent laboratories for studying cosmic rays (CRs), Galactic magnetic field, and AGN feedback in general. Using three-dimensional magnetohydrodynamic simulations that include relevant CR physics, I will show how leptonic AGN jets can explain the key characteristics of the Fermi bubbles and the spatially correlated features observed in the X-ray, microwave, and radio wavelengths. I will also discuss how we use our simulations in combination with the multi-wavelength data to obtain constraints on the composition of the Fermi bubbles.

Abstract: New physics can be light if it is hidden, coupling very weakly to the Standard Model. In this work we investigate the discovery prospects of Abelian hidden sectors in lower-energy fixed-target and high-precision experiments. We focus on a minimal supersymmetric realization consisting of an Abelian vector multiplet, coupled to hypercharge by kinetic mixing, and a pair of chiral Higgs multiplets. This simple theory can give rise to a broad range of experimental signals, including both commonly-studied patterns of hidden vector decay as well as new and distinctive hidden sector cascades. We find limits from the production of hidden states other than the vector itself. In particular, we show that if the hidden Abelian symmetry is higgsed, and the corresponding hidden Higgs boson has visible decays, it severely restricts the ability of the hidden sector to explain the anomalous muon magnetic moment.

Abstract: Can we learn about New Physics with astronomical and astro-particle data? Understanding how this is possible is key to unraveling one of the most pressing mysteries at the interface of cosmology and particle physics: the fundamental nature of dark matter. I will discuss some of the recent puzzling findings in cosmic-ray electron-positron data and in gamma-ray observations that might be related to dark matter. I will argue that cosmic-ray data, most notably from the AMS, Pamela and Fermi satellites, indicate that previously unaccounted-for powerful sources in the Galaxy inject high-energy electrons and positrons. Interestingly, this new source class might be related to new fundamental particle physics, and specifically to pair-annihilation or decay of galactic dark matter. This exciting scenario is directly constrained by Fermi gamma-ray observations, which also inform us on astrophysical source counterparts that could be responsible for the high-energy electron-positron excess. Observations of the gamma-ray emission from the central regions of the Galaxy as well as claims about a gamma-ray line at around 130 GeV also recently triggered a wide-spread interest: I will address the question of whether we are really observing signals from dark matter annihilation, how to test this hypothesis, and which astrophysical mechanisms constitute the relevant background.